ExpiryPager task holds HashTable mutexes for longer than necessary

The visitor used by the Expiry pager currently holds each hashTable lock for the duration of visiting every HashBucket owned by that lock. This unnecessarily increases the latency of front-end operations attempting to concurrently access the HashTable.

By releasing the hashTable lock after visiting each hash_bucket we could significantly reduce the effect the Defragmenter task has on front-end latency.

(See also: MB-14547).